The prior art in the present field falls generally within three areas, namely, the use of (a) current transformers, (b) voltage control techniques, and (c) variable resistance methods. Most of these are classified in U.S. Class 318, Sub-class 331.
With respect to the use of current transformers, the prior art includes the following:
U.S. Pat. No. 2,754,463 to Hansen which, basically, comprises a motor speed and torque controller.
U.S. Pat. No. 2,765,434 to Dudenhausen which discloses a speed control circuit having little emphasis in the current limiting area.
U.S. Pat. No. 3,295,040 to Schieman. This patent, entitled Current Limit Circuit, discloses the use of a three-phase transformer in conjunction with a DC power supply.
U.S. Pat. No. 3,526,819 to Graf. This patent, entitled Current Limit for Motor Control Systems, utilizes a phase controlled power amplifier in order to sense the point at which the armature current has decreased below a predetermined level, in order to thereby fire a control rectifier within the current limit circuit.
With respect to the use of voltage sensing means in current regulation, the prior art is represented by:
U.S. Pat. No. 3,253,204 to Hudson utilizing various voltage sensing techniques in order to produce an offset current under conditions at which the motor current would otherwise exceed an undesired level.
U.S. Pat. No. 3,583,947, entitled Full Wave Adjustable Motor Speed Control System with Feedback, discloses means for integrating the voltage appearing across the motor armature circuit, that is, balancing out the voltage drop due to the resistance of the motor armature in order to limit current levels.
With respect to the use of variable resistance means, the prior art is represented by the following:
U.S. Pat. No. 2,724,081 to De La Source. This patent utilizes current offset techniques in combination with a variable resistance approach in order to achieve current limitation.
U.S. Pat. No. 2,885,621 to Brown, entitled Current Regulating Circuits for Motor Control. This patent teaches use of a superimposed alternating current component upon a direct current component, together with the use of phase displacement techniques.
U.S. Pat. No. 3,305,720 to Safir. This patent discloses an arrangement of deceleration current amplifiers and deceleration relay coils in order to offset undesired current accelerations.
Other patents of interest in the present field include:
U.S. Pat. No. 3,412,307 to Welsh, entitled Current Limiting Motor Control Circuit. This patent comprises a means for acceleration and deceleration in bi-directional motors.
U.S. Pat. No. 3,551,774 to Rusch. This patent makes use of a voltage across an impedance and, additionally, utilizes voltage spikes as control means. Additionally, thermal sensing is involved.
U.S. Pat. No. 3,710,213 to Hammer, relates to pulse-rate control of motor speed.
U.S. Pat. No. 3,868,554 to Conrad. This patent utilizes temperature sensing by thyristors in order to limit motor current.
It is to be noted that the most common method of regulating load current through a DC motor is to insert a resistance in series with the load. In such a method, the power supply is divided between the load and the series resistor, with the resultant disadvantage of system inefficiency.
The more desirable method of controlling DC motors in DC loads is by voltage control, generally known as average voltage controlling. In this method the DC load voltage is pulsed on and off either by a switch or by a solid state device. The variation and time duration during which the switch is in the on and off positions provide control over the average load, voltage, or motor speed. This system places full voltage across the motor for a set period of time regardless of the turn peaks. The load current, when the motor is stalled or when peak load conditions are present, is limited only by load impedance. In this system, the period of the voltage pulses is usually constant and is set by a multivibrator; the ratio of on to off time therein may be varied through the use of a referenced potentiometer which changes the RC time constant of the vibrator.
Despite the general acceptance of the above method, it is, nonetheless, in many instances, desirable to achieve an absolute regulation of the current flow through the motor in order to achieve not only motor control but to maximize the life of the motor as well as the associated circuitry. Also, lack of sensitivity control in current regulation has been a problem in the multivibrator approach.
Current regulation in DC motors is particularly important in the field of vehicle drive in which regulation of the current drawn from the battery must be subject to precise control. Additionally, current regulation is necessary in order to achieve accurate torque.
The present invention may be viewed as response to problems of motor heating and battery drain which, generally, are the result of undesirably high rms values within the load current.